Astrum Dragynn

Immutable Form. The Dragynn is immune to any spell or effect that would alter its form.

Magic Resistance. The Dragynn has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Dragynn's weapon attacks are magical.

Legendary Resistance (3/Day). If the Dragynn fails a saving throw, it can choose to succeed instead.

Heat Levels. The Dragynn has levels of heat. When the Dragynn is at 4 levels of heat, it cannot take legendary actions, temporarily loses all damage resistances, and gains vulnerability to fire, cold, acid, thunder, piercing, bludgeoning and slashing damage. 

Overheat. Whenever the Dragynn takes the attack action, or takes 25 points of fire damage in one round, it gains one level of heat. 

Multiattack. The Dragynn can make three attacks: two with its Rending Claws and one with its Irontail Slam. 

Irontail Slam. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage.

Steel Claws. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 23 (3d10 + 7) slashing damage.

Expel Heat. In order to use this ability, the Dragynn must possess at least one level of heat. The Dragynn resets its heat level back to 0, and all creatures within 30 feet of it must make a DC 17 Dexterity saving throw or take fire damage equal to ten times the levels of heat lost this turn. 

Exhaust Breath (Recharge 6). In order to use this ability, the Dragynn must possess at least one level of heat, which it consumes when this ability is used. The Dragynn exhales flame in a 60-foot cone. Each creature in that area must make a DC 17 Dexterity saving throw, taking 45 (10d8) fire damage on a failed save, or half as much damage on a successful one.

The Dragynn can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Dragynn regains spent legendary actions at the start of its turn.

Coolant. The Dragynn reduces its heat level by 1.

Dual Rend. The Dragynn makes two attacks with its Steel Claws. 

Metal Menders (Costs 2 Actions). The Dragynn regains 8d8 hit points and gains a level of heat. 

Overdrive (Costs 3 Actions). The Dragynn doubles all of its movement speeds and takes two actions until the end of its next turn, and generates a level of heat.